The initial discovery of x-rays by Röntgen in 1895 [W. C. Röntgen, “Eine Neue Art von Strahlen (Würzburg Verlag, 1896); “On a New Kind of Rays,” Nature, Vol. 53, pp. 274-276 (Jan. 23, 1896)] occurred by accident when Röntgen was experimenting with electron bombardment of targets in vacuum tubes. These high energy, short wavelength photons are now routinely used for medical applications and diagnostic evaluations, as well as for security screening, industrial inspection, quality control and failure analysis, and for scientific applications such as crystallography, tomography, x-ray fluorescence analysis and the like.
The laboratory x-ray source was later improved by Coolidge in the early 20th century [see, for example, William D. Coolidge, U.S. Pat. No. 1,211,092, issued Jan. 2, 1917, U.S. Pat. No. 1,917,099, issued Jul. 4, 1933, and U.S. Pat. No. 1,946,312, issued Feb. 6, 1934], and, later in the 20th century, systems generating very intense beams of x-rays using synchrotrons or free electron lasers (FELs) have been developed. These synchrotron or FEL systems, however, are physically very large systems, requiring large buildings and acres of land for their implementation. For compact, practical lab-based systems and instruments, most x-ray sources today still use the fundamental mechanism of the Coolidge tube.
An example of the simplest x-ray source, a transmission x-ray source 08, is illustrated in FIG. 1 The source comprises a vacuum environment (typically 10−6 torr or better) commonly provided by a sealed vacuum tube 02 or active pumping, manufactured with sealed electrical leads 21 and 22 that pass from the negative and positive terminals of a high voltage source 10 outside the tube to the various elements inside the vacuum tube 02. The source 08 will typically comprise mounts 03 which secure the vacuum tube 02 in a housing 05, and the housing 05 may additionally comprise shielding material, such as lead, to prevent x-rays from being radiated by the source 08 in unwanted directions.
Inside the vacuum tube 02, an emitter 11 connected through the lead 21 to the high voltage source 10 serves as a cathode and generates a beam of electrons 111, often by running a current through a filament. The target 01 is electrically connected to the opposite high voltage lead 22, and therefore serves as an anode. The emitted electrons 111 accelerate towards the target 01 and collide with it at high energy, with the energy of the electrons determined by the magnitude of the accelerating voltage. The collision of the electrons 111 into the solid target 01 induces several effects, including the generation of x-rays 888, some of which exit the vacuum tube 02 through a window 04 designed to transmit x-rays. In the configuration shown in FIG. 1, the target 01 is deposited or mounted directly on the window 04 and the window 04 forms a portion of the wall of the vacuum chamber. In other prior art embodiments, the target may be formed as an integral part of the window 04 itself.
Another example of a common x-ray source design is the reflection x-ray source 80, is illustrated in FIG. 2. Again, the source comprises a vacuum environment (typically 10−6 torr or better) commonly maintained by a sealed vacuum tube 20 or active pumping, and manufactured with sealed electrical leads 21 and 22 that pass from the negative and positive terminals of a high voltage source 10 outside the tube to the various elements inside the vacuum tube 20. The source 80 will typically comprise mounts 30 which secure the vacuum tube 20 in a housing 50, and the housing 50 may additionally comprise shielding material, such as lead, to prevent x-rays from being radiated by the source 80 in unwanted directions.
Inside the tube 20, an emitter 11 connected through the lead 21 to the high voltage source 10 serves as a cathode and generates a beam of electrons 111, often by running a current through a filament. A target 100 supported by a target substrate 110 is electrically connected to the opposite high voltage lead 22 and target support 32, and therefore serves as an anode. The electrons 111 accelerate towards the target 100 and collide with it at high energy, with the energy of the electrons determined by the magnitude of the accelerating voltage. The collision of the electrons 111 into the target 100 induces several effects, including the generation of x-rays, some of which exit the vacuum tube 20 and are transmitted through a window 40 that is transparent to x-rays.
In an alternative prior art embodiment for a reflective x-ray source (not shown in FIG. 2), the target 100 and substrate 110 may be integrated or comprise a solid block of the same material, such as copper (Cu). Also not shown in FIGS. 1 and 2, but commonly employed in practice, electron optics (electrostatic or electromagnetic lenses) may be provided to guide and shape the path of the electrons, forming a more concentrated, focused beam at the target. Likewise, electron sources comprising multiple emitters may be provided to provide a larger, distributed source of electrons.
When the electrons collide with a target 100, they can interact in several ways. These are illustrated in FIG. 3. The electrons in the electron beam 111 collide with the target 100 at its surface 102, and the electrons that pass through the surface transfer their energy into the target 100 in an interaction volume 200, generally defined by the incident electron beam footprint (area) times the electron penetration depth. For an incident electron beam of very small size (e.g. a beam diameter <100 nm) the interaction volume 200 is typically “pear” or “teardrop” shaped in three dimensions, and symmetric around the electron propagation direction. For a larger beam, the interaction volume will be represented by the convolution of this “teardrop” shape with the lateral beam intensity profile.
An equation commonly used to estimate the penetration depth for electrons into a material is Potts' Law [P. J. Potts, Electron Probe Microanalysis, Ch. 10 of A Handbook of Silicate Rock Analysis, Springer Netherlands, 1987, p. 336)], which states that the penetration depth x in microns is related to the 10% of the value of the electron energy E0 in keV raised to the 3/2 power, divided by the density of the material:
                              x          ⁡                      (            μm            )                          =                  0.1          ×                                    E              0              1.5                        ρ                                              [                  Eqn          .                                          ⁢          1                ]            For less dense material, such as a diamond substrate, the penetration depth is much larger than for a material with greater density, such as most elements used for x-ray generation.
There are several energy transfer mechanisms that can occur. Throughout the interaction volume 200, electron energy may simply be converted into heat. Some absorbed energy may excite the generation of secondary electrons, typically detected from a region 221 located near the surface, while some electrons may also be backscattered, which, due to their higher energy, can be detected from a somewhat larger region 231.
Throughout the interaction volume 200, including in the regions 221 and 231 near the surface and extending approximately 3 times deeper into the target 100, x-rays 888 are generated and radiated outward in all directions. The x-ray radiation can have a complex energy spectrum. As the electrons penetrate the material, they decelerate and lose energy, and therefore different parts of the interaction volume 200 produce x-rays with different properties. A typical x-ray radiation spectrum for radiation from the collision of 100 keV electrons with a tungsten target is illustrated in FIG. 4.
As shown in FIG. 4, the broad spectrum x-ray radiation 388 arises from electrons that were diverted from their initial trajectory, depending on how close they pass to various nuclei and other electrons. The reduction in electron energy and the change momentum associated with the change in direction generate the radiation of x-rays. Because a wide range of deflections and decelerations can occur, due to the proximity statistics of the electron collisions with the atoms of the target material, the change in energy is a continuum, and therefore, the energy of the generated x-rays also is a continuum. Greater radiation occurs at the low end of the energy spectrum, with far less occurring at higher energy, and reaching an absolute limit of no x-rays with energy larger than the original electron energy (in this example, 100 keV). Due to their origin in deceleration of electrons, this kind of continuum x-ray radiation 388 is commonly called bremsstrahlung, after the German word “bremsen” for “braking”.
These continuum x-rays 388 are generated throughout the interaction volume, shown in FIG. 3 as the largest shaded portion 288 of the interaction volume 200. At lower energy, the bremsstrahlung x-rays 388 are typically radiated isotropically, i.e. with little variation in intensity with radiation direction [see, for example, D. Gonzales, B. Cavness, and S. Williams, “Angular distribution of thick-target bremsstrahlung produced by electrons with initial energies ranging from 10 to 20 keV incident on Ag”, Phys. Rev. A, vol. 84, 052726 (2011)], higher energy excitation can have increased radiation normal to the electron beam, i.e. at “0 degrees” for an incident beam at 90 degrees with respect to the target surface. [See, for example, J. G. Chervenak and A. Liuzzi, “Experimental thick-target bremsstrahlung spectra from electrons in the range 10 to 30 kev”, Phys. Rev. A, vol. 12(1), pp. 26-33 (July, 1975).]
As was shown in FIGS. 1 and 2, the x-ray source 08 or 80 will typically have a window 04 or 40. This window 04 or 40 may additionally comprise a filter, such as a sheet or layer of aluminum, that attenuates the low energy x-rays, producing the modified energy spectrum 488 shown in FIG. 4.
When the electron energy is larger than the binding energy of an inner-shell (core-shell) electron of an element within the target, ejection of the electron (ionization) from the shell may occur, creating a vacancy. Electrons from less strongly bound outer shell(s) are then free to transition to the vacant inner shell, filling the vacancy. As the filling electron moves down to the lower energy level, the excess energy is radiated in the form of an x-ray photon. This is known as “characteristic” radiation because the energy of the photon is characteristic of the chemical element that generates the photon.
In the example shown in FIG. 4, an electron of 100 keV may ionize a K-shell electron of a tungsten atom, which has a binding energy of 69.5 keV. If the vacancy is filled by an electron from the L-shell, which has a binding energy of 10.2 keV, the x-ray photon has an energy equal to the energy difference between these two levels, or Kα1=59.3 keV. Likewise, a transition from the M-shell to the K-shell is denoted as Kβ1=67.2 keV. Splittings can occur in the various levels, giving rise to slight variations in energy, e.g. Kβ1, Kβ2, Kβ3 etc.
Because these discrete spectral lines depend on the atomic structure of the target material, the radiation is generally called “characteristic lines”, since they are a characteristic of the particular material. The sharp lines 988 in the example of an x-ray radiation spectrum shown in FIG. 4 are “characteristic lines” for tungsten. Individual characteristic lines can be quite bright, and may be monochromatized with an appropriate filter or crystal monochromator where a monochromatic source is desired. The relative x-ray intensity (flux) ratio of the characteristic line(s) to the bremsstrahlung radiation depends on the element and the incident electron energy, and can vary substantially. In general, a maximum ratio for a given target material is obtained when the incident electron energy is 3 to 5 times the ionization energy of the inner shell electrons.
Returning to FIG. 3, these characteristic x-rays 388 are primarily generated in a fraction of the electron penetration depth, shown as the second largest shaded portion 248 of the interaction volume 200. The relative depth is influenced in part by the energy of the electrons 111, which typically falls off with increasing depth. If the electron energy does not exceed the binding energy for electrons within the target, no characteristic x-rays will be generated at all. The greatest radiation of characteristic lines may occur under bombardment with electrons having three to five times the energy of the characteristic x-ray photons. Because these characteristic x-rays result from atomic transitions between electron shells, the radiation will generally be entirely isotropic. The actual dimensions of this interaction volume 200 may vary, depending on the energy and angle of incidence of the electrons, the surface topography and other properties (including local charge density), and the density and atomic composition of the target material.
For some applications, broad-spectrum x-rays may be appropriate. For other applications, a monochromatic source may be desired or even necessary for the sensitivity or resolution required. In general, the composition of the target material is selected to provide x-ray spectra with ideal characteristics for a specific application, such as strong characteristic lines at particular wavelengths of interest, or bremsstrahlung radiation over a desired bandwidth.
Control of the x-ray radiation properties of a source may be governed by the selection of an electron energy (typically changed by varying the accelerating voltage), x-ray target material selection, and by the geometry of x-ray collection from the target.
Although the x-rays may be radiated isotropically, as was illustrated in FIG. 3, only the x-ray radiation 888 within a small solid angle in the direction of window 440 in the source, as shown in FIGS. 5A-C, will be collected. The x-ray brightness, (also called “brilliance” by some), defined as the number of x-ray photons per second per solid angle in mrad2 per area of the x-ray source in mm2 (some measures may also include a bandwidth window of 0.1% in the definition), is an important figure-of-merit for a source, as it relates to obtaining good signal-to-noise ratios for downstream applications.
The brightness can be increased by adjusting the geometric factors to maximize the collected x-rays. As illustrated in FIGS. 5A-C, the surface of a target 100 in a reflection x-ray source is generally mounted at an angle θ (as was also shown in FIG. 2) and bombarded by a distributed electron beam 111. X-ray radiation through a window 440 is shown for a set of five equally spaced radiation spots 408 for three target angles: θ=60° in FIG. 5A, θ=45° in FIG. 5B, and θ=30° in FIG. 5C. For a source at a high angle θ, for a solid angle centered at the window 440, the five spots are more spread out and brightness is reduced, while for low angle θ, the five source spots appear to be closer together, thus radiating more x-rays into the same solid angle and resulting in an increased brightness.
In principle, it may appear that a source mounted at θ=0° would have all sources apparently overlapping, accumulating the generated x-rays, and therefore would have the largest possible brightness. In practice, radiation at 0° occurs parallel to the surface of a solid metal target for conventional sources, and since the x-rays must propagate along a long length of the target material before emerging, most of the produced x-rays will be attenuated (reabsorbed) by the target material, reducing brightness. In practice, a source with take-off angle of around 6° to 15° (depending on the source configuration, target material, and electron energy) will often provide the greatest practical brightness, concentrating the apparent size of the source while reducing re-absorption within the target material and is therefore commonly used in commercial x-ray sources.
The effective source area is the projected area viewed along the direction along which x-ray are collected for use, i.e. along the axis of the x-ray beam. Because of the limited electron penetration depth, the effective source area for an incident electron beam with a size comparable or larger than the electron penetration depth is dependent on the angle between the axis of the x-ray beam and the surface of the target, referred to as the “take-off angle”. When the electron beam size is much larger than the electron penetration depth, the effective source area decreases with decreasing take-off angle. This effect has been used to increase x-ray source brightness. However, with an extensive flat target, there is a limit to this benefit, due to the increasing absorption of x-rays from their production points inside the target as they propagate to the surface, which increases with a smaller take-off angle. Typically, a compromise between improved brightness from a lower angle and reduced brightness from reabsorption is reached around a take-off angle of ˜6 degrees.
Another way to increase the brightness of the x-ray source for bremsstrahlung radiation is to use a target material with a higher atomic number Z, as efficiency of x-ray production for bremsstrahlung radiation scales with increasingly higher atomic number materials. Furthermore, the x-ray radiating material should ideally have good thermal properties, such as a high melting point and high thermal conductivity, in order to allow higher electron power loading on the source to increase x-ray production. For these reasons, targets are often fabricated using tungsten, with an atomic number Z=74. Table I lists several materials that are commonly used for x-ray targets, several additional potential target materials (notably useful for specific characteristic lines of interest), and some materials that may be used as substrates for target materials. Melting points, and thermal and electrical conductivities are presented for values near 300° K (27° C.). Most values are taken
TABLE IVarious Target and Substrate Materials and Selected Properties.AtomicMeltingThermalElectricalMaterialNumberPoint ° C.ConductivityConductivity(Elemental Symbol)Z(1 atm)(W/(m ° C.))(MS/m)Common Target Materials:Chromium (Cr)24190793.77.9Iron (Fe)26153880.210.0Cobalt (Co)27149510017.9Copper (Cu)29108540158.0Molybdenum (Mo)42262313818.1Silver (Ag)4796242961.4Tungsten (W)74342217418.4Other Possible Target Materials:Titanium (Ti)22166821.92.6Gallium (Ga)353040.67.4Rhodium (Rh)45196415023.3Indium (In)4915781.612.5Cesium (Cs)552835.94.8Rhenium (Re)75318547.95.8Gold (Au)79106431744.0Lead (Pb)8232735.34.7Other Potential Substrate Materials with low atomic number:Beryllium (Be) 4128720026.6Carbon (C): Diamond 6*230010−19Carbon (C): Graphite ∥ 6*19500.25Carbon (C): 6*3180100.0Nanotube (SWNT)Carbon (C): 6*200Nanotube (bulk)Boron Nitride (BN)B = 5**2010−17N = 7Silicon (Si)1414141241.56 × 10−9Silicon Carbide (β-SiC)Si = 1427980.4910−9 C = 6Sapphire (Al2O3) ∥ CAl = 13205332.510−20O = 8*Carbon does not melt at 1 atm; it sublimes at ~3600° C.**BN does not melt at 1 atm; it sublimes at ~2973° C.from the CRC Handbook of Chemistry and Physics, 90th ed. [CRC Press, Boca Raton, Fla., 2009]. Other values are taken from various sources found on the Internet. Note that, for some materials, such as sapphire for example, thermal conductivities an order of magnitude larger may be possible when cooled to temperatures below that of liquid nitrogen (77° K) [see, for example, Section 2.1.5, Thermal Properties, of E. R. Dobrovinskaya et al., Sapphire: Material, Manufacturing, Applications, Springer Science+Business Media, LLC (2009)].
Other ways to increase the brightness of the x-ray source are: increasing the electron current density, either by increasing the overall current or by focusing the electron beam to a smaller spot using, for example, electron optics; or by increasing the electron energy by increasing the accelerating voltage (which increases x-ray production per unit electron energy deposited in the target, and may excite more radiation in the characteristic lines as well).
However, these improvements have a limit, in that all can increase the amount of heat generated in the interaction volume. The problem is exacerbated by having the target in a vacuum, so no air cooling from the surface by convection may occur. If too much heat is generated within the target, the target material may undergo phase changes, even as far as melting or evaporating. Because the vast majority of the energy deposited into the target by an electron beam becomes heat, thermal management techniques are an important tool for building better x-ray sources.
One prior art technology that has been developed to address this problem is the rotating anode system, illustrated in FIGS. 6A and 6B. In FIG. 6A, a cross-section is shown for a rotating anode x-ray source 580 comprising a target anode 500 that typically rotates between 3,300 and 10,000 rpm. The target anode 500 is connected by a shaft 530 to a rotor 520 supported by conducting bearings 524 that connect, through its mount 522, to the lead 22 and the positive terminal of the high voltage supply 10. The rotation of the rotor 520, shaft 530 and anode 500, all within the vacuum chamber 20, is typically driven inductively by stator windings 525 mounted outside the vacuum.
The surface of the target anode 500 is shown in more detail in FIG. 6B. The edge 510 of the rotating target anode 500 is sometimes beveled at an angle, and the source of the electron beam 511 is in a position to direct the electron beam onto the beveled edge 510 of the target anode 500, generating x-rays 888 from a target spot 501. As the target spot 501 generates x-rays, it heats up, but as the target anode 500 rotates, the heated spot moves away from the target spot 501, and the electron beam 511 now irradiates a cooler portion of the target anode 500. The hot spot has the time of one rotation to cool before becoming heated again when it passes through the hot spot 501. By continuously rotating the target anode 500, x-rays are generated from a fixed single spot, while the total area of the target illuminated by the electron beam is substantially larger than the electron beam spot, effectively spreading the electron energy deposition over a larger area (and volume).
Another approach to mitigating heat is to use a target with a thin layer of target x-ray generating material deposited onto a substrate with high heat conduction. Because the interaction volume is thin, for electrons with energies up to 100 keV the target material itself need not be thicker than a few microns, and can be deposited onto a substrate, such as diamond, sapphire or graphite that conducts the heat away quickly. However, as noted in Table I, diamond is a very poor electrical conductor, so the design of any anode fabricated on a diamond substrate must still provide an electrical connection between the target material of the anode and the positive terminal of the high voltage. [Diamond mounted anodes for x-ray sources have been described by, for example, K. Upadhya et al. U.S. Pat. No. 4,972,449; B. Spitsyn et. al. U.S. Pat. No. 5,148,462; and M. Fryda et al., U.S. Pat. No. 6,850,598].
The substrate may also comprise channels for a coolant, for example liquids such as water or ethylene glycol, or a gas such as hydrogen or helium, that remove heat from the substrate [see, for example, Paul E. Larson, U.S. Pat. No. 5,602,899]. Water-cooled anodes are used for a variety of x-ray sources, including rotating anode x-ray sources.
The substrate may in turn be mounted to a heat sink comprising copper or some other material chosen for its thermally conducting properties. The heat sink may also comprise channels for a coolant, to transport the heat away [see, for example, Edward J. Morton, U.S. Pat. No. 8,094,784]. In some cases, thermoelectric coolers or cryogenic systems have been used to provide further cooling to an x-ray target mounted onto a heat sink, again, all with the goal of achieving higher x-ray brightness without melting or damaging the target material through excessive heating.
Another approach to mitigating heat for microfocus sources is to use a target created by a jet of liquid metal. Electrons bombard a conducting jet of liquid gallium (Z=31), and because the heated gallium flows away from the electron irradiation volume with the jet, higher current densities are possible. [See, for example, M. Otendal, et al., “A 9 keV electron-impact liquid-gallium-jet x-ray source”, Rev. Sci. Instrum., vol. 79, 016102, (2008)].
Although effective in certain circumstances, there is still room for improvement. Jets of liquid metal require an elaborate plumbing system and consumables, are limited in the materials (and thus values of Z and their associated spectra) that may be used, and are difficult to scale to larger output powers. In the case of thin film targets of uniform solid material coated onto diamond substrates, there is still a limitation in the amount of heat that can be tolerated before damage to the film may occur, even if used in a rotating anode configuration. Conduction of heat only occurs through the bottom of the film. In a lateral dimension, the same conduction problem exists as exists in the bulk material.
There is therefore a need for an x-ray source that may be used to achieve higher x-ray brightness through the use of a higher electron current density, but that is still compact enough to fit in a laboratory or table-top environment, or even be useful in portable devices. Such brighter sources would enable x-ray based tools that offer better signal to noise ratios for imaging and other scientific and diagnostic applications.